Yes you can transfer only your main if you wish, there shouldn't be any problem transferring back, just need to wait for the transfer cooldown, no need for any gold passes. Anything in your delivery box and probably auction house will be lost, so make sure that's cleared out. It doesn't cost you any extra money to also transfer all of your mules in the same transfer, so you may consider transferring them over just in case you make some good friends on your new server and decide to stay there.